This invention relates generally to gas-fired condensing furnaces. More specifically, the present invention relates to a corrosion resistant condensing heat exchanger for use in the corrosive environment of a gas-fired condensing furnace and to the method of manufacture thereof.
Due to the cost and shortage of natural gs, attempts have been made to design and construct more efficient gas-fired hot air furnaces. One method for maximizing the heat energy transferred from a heating fluid, i.e. combustion gas, to air to be heated, i.e. the air in the enclosure or space to be heated, is to transer as much latent heat as possible from the water vapor in the heating fluid to the air to be heated. Thus, increases in furnace heating efficiency have been accomplished by cooling the heating fluid, while still within the furnace, to below the dew point to recover some of the latent heat of vaporization as usable energy. This is generally accomplished by adding a condensing heat exchanger to the primary heat exchanger, and passing air to be heated initially over the condensing heat exchanger, and then over the primary heat exchanger. Depending on the type of condensing furnace, efficiencies can be in the low-to-mid 90% range.
Some furnace heat exchangers have been constructed from two engineering metal sheets such that a fluid flow path is created when the two sheets are stamped and assembled. This type of heat exchanger is known as a clamshell heat exchanger. The corrosive environment of a condensing heat exchanger, which may have a variety of acids, including H.sub.2 SO.sub.4 or HCL, necessitates different material requirements that those typical of the primary heat exchanger. Concentrations of as little as 10 ppm (parts per million) of H.sub.2 SO.sub.4 or HCL may severely corrode bare steel and pit aluminum and copper. Accordingly, a condensing heat exchanger must be constructed of material having good heat transfer, adequate strength, minimum material thickness, resistance to chemical attack, and low manufacturing costs. Due to the material requirements for the corrosive environment of a condensing heat exchanger, these heat exchangers are generally manufactured from 300 Series stainless steel which is more costly than carbon steels or other engineering metals previously associated with furnace heat exchanger construction. Organic coatings on carbon steels, which are applied from a liquid or powder state, perform very poorly when used in condensing heat exchangers. Such coatings inherently contain voids thereby causing localized corrosion of the steel substrate. Sheet coatings of polymeric material laminated on solid sheets of metals are free of voids and withstand the fabricating process, but after furnace operation blisters are formed on the laminating surface. The blisters are formed by the coalescence of water molecules that permeate through the polymer film and become trapped at the film-steel substrate interface. These blisters could impede the passage of flue gas and increase the pressure drop across the heat exchanger, thus causing the furnace to operate improperly.